The deregulation of the ubiquitin-proteasome system (UPS) is a hallmark feature of many neurodegenerative disorders, including retinal degenerations, which involve misfolding and deregulation of protein turnover in photoreceptors. Our long-range objective is to understand the role of a regulatory complex of the UPS- chaperone machinery in controlling substrate degradation and folding and use this information to identify targets for novel therapeutic strategies, which presently lack efficacy toward selective biological and disease processes of the UPS. The goal of this proposal is to understand the molecular and intracellular mechanisms that sustain the balance between turnover and biogenesis of proteins, two tightly coupled processes that are vital for retinal-neuron function and survival and that are implicated in the pathogenesis of a wide variety of neuro-visual disorders of the human. Achievement of this goal will allow us and others to identify novel pharmacological targets and develop therapeutic strategies to manipulate the UPS-chaperone machinery, prevent the toxic deregulation of normal or expression of mutated proteins under various retinal disease states and aging manifestations causing the impairment of the UPS-chaperone machinery and neuro-visual function. The proposed research builds upon our prior research findings which demonstrated that substrates are presented to the proteasome upon synthesis and that the activity of the proteasome is regulated and compartmentalized in the cell. Ran-binding protein 2 (RanBP2) via two distinct domains, the cyclophilin-like (CLD) and cyclophilin (CY) domains, is a key controller of such processes. This proposal tests our overall hypothesis that RanBP2 and its CLD and CY domains control two tightly coupled processes, the UPS-dependent turnover and biogenesis of selective proteins that are critical to the function and survival of selective retinal neurons and retinal pigment epithelium (RPE) cells under normal and pathophysiological conditions. We will test this hypothesis by accomplishing the following specific aims, which focus on the mechanistic roles of RanBP2, and its CLD and CY domains, in the regulation of protein homeostasis and survival of selective retinal neurons under normal and disease stress conditions. Aim 1. Test the hypothesis that loss-of-function of RanBP2 has differential biological and pathological effects among retinal neurons, some of which may be essential to the viability or protein homeostasis of such neurons. Aim 2. Test the hypothesis that selective loss-of-function of the CLD activity of RanBP2 up-regulates selective functions of the UPS that are critical to homeostasis of selective retinal proteins, function and survival of retinal neurons and RPE cells. Aim 3. Test the hypothesis that the cis-trans prolyl isomerase (PPIase) activity in the CY domain of RanBP2 controls the biogenesis of selective proteins that are critical to the function or survival of retinal neurons and RPE cells.